In a typical prior art head and disk system a magnetic transducer is supported by the suspension as it flies above the disk. The magnetic transducer, usually called a “head” or “slider” is composed of elements that perform the task of writing magnetic transitions (the write head ) and reading the magnetic transitions (the read head). The disk has a set of thin films that include a ferromagnetic material in which the magnetic transitions are recorded. The disk is attached to a spindle that is driven by a spindle motor to rotate the disk under the slider which is supported on an actuator that positions the slider over selected points as the disk rotates under the slider. During the manufacturing process permanent servo information is written in the magnetic material to allow the disk drive control system to determine which track the slider is over, a relative position inside the track and index information on the rotational position of the disk. The accuracy of the placement of the servo information is one of the limiting factors in the density of data which can be written and retrieved by the disk drive.
In U.S. Pat. No. 5,119,248 to Bizjak, et al., a servo disk having calibration tracks is described. First and second servo lines are written on opposite sides of the disk is utilized. Separate servo heads are disposed on each side of the disk. It is necessary to know the exact relationship between the intersection of first and second servo heads and the distance to the center of each data head. Ideally the servo heads will detect their respective servo lines simultaneously at a servo line intersection. However, if there is offset introduced between the heads, such simultaneous detection will not occur. The calibration track has a nominal center at a known set of servo line intersections. The calibration track, in one embodiment, utilizes an amplitude comparison scheme to position the data heads. When a data head is positioned over its calibration track, any variation in the spacing of the center of that data head to the intersection of the two servo heads will be indicated by a time difference in the occurrence of detection in the first and second servo lines respectively.
The conventional method for Servo Track Write calibration and tester to tester variance measurements is to write magnetic servo tracks and send the disk to the lab for chemical ferro-fluid application and microscope inspection indexing to measure track zero position. Some of the problems with this method are that it is slow (about 30 minutes per disk), it requires chemicals, and it has low resolution of 0.1 mm (approximately 100 tracks).